Single-access Key - Diagnostic ('artificial') Versus Synoptic ('natural') Keys

Diagnostic ('artificial') Versus Synoptic ('natural') Keys

Any single-access key organizes a large set of items into a structure that breaks them down into smaller, more accessible subsets, with many keys leading to the smallest available classification unit (a species or infraspecific taxon). However, a trade-off exists between keys that concentrate on making identification most convenient and reliable (diagnostic keys), and keys which aim to reflect the scientific classification of organisms (synoptic keys). The first type of keys limits the choice of characteristics to those most reliable, convenient, and available under certain conditions. Multiple diagnostic keys may be offered for the same group of organisms: Diagnostic keys may be designed for field (field guides) or laboratory use, for summer or winter use, and they may use geographic distribution or habitat preference of organisms as accessory characteristics. They do so at the expense of creating artificial groups in the key.

An example of a diagnostic key is shown below. It is not based on the taxonomic classification of the included species — compare with the botanical classification of oaks.

In contrast, synoptic keys follow the taxonomic classification as close as possible. Where the classification is already based on phylogenetic studies, the key represents the evolutionary relationships within the group. To achieve this, these keys often have to use more difficult characteristics, which may not always be available in the field, and which may require instruments like a hand lens or microscope. Because of convergent evolution, superficially similar species may be separated early in the key, with superficially different, but genetically closely related species being separated much later in the key. Synoptic keys are typically found in scientific treatments of a taxonomic group ("monographs"). Despite being less useful for field identification

An example of a synoptic key (corresponding to the diagnostic key shown below) is shown further below. In plants, flower and fruit characteristics often are important for primary taxonomic classification:

Example of a diagnostic dichotomous key for some eastern United States oaks based on leaf characteristics

1. Leaves usually without teeth or lobes: 2
1. Leaves usually with teeth or lobes: 5
2. Leaves evergreen: 3
2. Leaves not evergreen: 4
3. Mature plant a large tree — Southern live oak Quercus virginiana
3. Mature plant a small shrub — Dwarf live oak Quercus minima
4. Leaf narrow, about 4-6 times as long as broad — Willow oak Quercus phellos
4. Leaf broad, about 2-3 times as long as broad — Shingle oak Quercus imbricaria
5. Lobes or teeth bristle-tipped: 6
5. Lobes or teeth rounded or blunt-pointed, no bristles: 7
6. Leaves mostly with 3 lobes — Blackjack oak Quercus marilandica
6. Leaves mostly with 7-9 lobes — Northern red oak Quercus rubra
7. Leaves with 5-9 deep lobes — White oak Quercus alba
7. Leaves with 21-27 shallow lobes — Swamp chestnut oak Quercus prinus

Example of a synoptic (taxonomic) dichotomous key for some eastern United States oaks, reflecting taxonomic classification

1. Styles short; acorns mature in 6 months, sweet or slightly bitter, inside of acorn shell hairless (Quercus sect. Quercus, white oaks): 2
1. Styles long, acorns mature in 18 months, very bitter, inside of acorn shell woolly (Quercus sect. Lobatae, red oaks): 5
2. Leaves evergreen: 3
2. Leaves not evergreen: 4
3. Mature plant a large tree — Southern live oak Quercus virginiana
3. Mature plant a small shrub — Dwarf live oak Quercus minima
4. Leaves with 5-9 deep lobes — White oak Quercus alba
4. Leaves with 21-27 shallow lobes — Swamp chestnut oak Quercus prinus
5. Leaves usually without teeth or lobes: 6
5. Leaves usually with teeth or lobes: 7
6. Leaf narrow, about 4-6 times as long as broad — Willow oak Quercus phellos
6. Leaf broad, about 2-3 times as long as broad — Shingle oak Quercus imbricaria
7. Leaves mostly with 3 lobes — Blackjack oak Quercus marilandica
7. Leaves mostly with 7-9 lobes — Northern red oak Quercus rubra

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